.TH STAB 8 "31 October 2011" iproute2 Linux
.
.SH NAME
tc\-stab \- Generic size table manipulations
.
.SH SYNOPSIS
.nf
tc qdisc add ... stab
.RS 4
[ \fBmtu\fR BYTES ] [ \fBtsize\fR SLOTS ]
[ \fBmpu\fR BYTES ] [ \fBoverhead\fR BYTES ]
[ \fBlinklayer\fR { adsl | atm | ethernet } ] ...
.RE
.fi

.SH OPTIONS
For the description of BYTES \- please refer to the \fBUNITS\fR
section of \fBtc\fR(8).

.IP \fBmtu\fR 4
.br
maximum packet size we create size table for, assumed 2048 if not specified explicitly
.IP \fBtsize\fR
.br
required table size, assumed 512 if not specified explicitly
.IP \fBmpu\fR
.br
minimum packet size used in computations
.IP \fBoverhead\fR
.br
per\-packet size overhead (can be negative) used in computations
.IP \fBlinklayer\fR
.br
required linklayer specification.
.PP
.
.SH DESCRIPTION
.
Size tables allow manipulation of packet sizes, as seen by the whole scheduler
framework (of course, the actual packet size remains the same). Adjusted packet
size is calculated only once \- when a qdisc enqueues the packet. Initial root
enqueue initializes it to the real packet's size.

Each qdisc can use a different size table, but the adjusted size is stored in
an area shared by whole qdisc hierarchy attached to the interface. The effect is
that if you have such a setup, the last qdisc with a stab in a chain "wins". For
example, consider HFSC with simple pfifo attached to one of its leaf classes.
If that pfifo qdisc has stab defined, it will override lengths calculated
during HFSC's enqueue; and in turn, whenever HFSC tries to dequeue a packet, it
will use a potentially invalid size in its calculations. Normal setups will
usually include stab defined only on root qdisc, but further overriding gives
extra flexibility for less usual setups.

The initial size table is calculated by \fBtc\fR tool using \fBmtu\fR and
\fBtsize\fR parameters. The algorithm sets each slot's size to the smallest
power of 2 value, so the whole \fBmtu\fR is covered by the size table. Neither
\fBtsize\fR, nor \fBmtu\fR have to be power of 2 value, so the size
table will usually support more than is required by \fBmtu\fR.

For example, with \fBmtu\fR\~=\~1500 and \fBtsize\fR\~=\~128, a table with 128
slots will be created, where slot 0 will correspond to sizes 0\-16, slot 1 to
17\~\-\~32, \&..., slot 127 to 2033\~\-\~2048. Sizes assigned to each slot
depend on \fBlinklayer\fR parameter.

Stab calculation is also safe for an unusual case, when a size assigned to a
slot would be larger than 2^16\-1 (you will lose the accuracy though).

During the kernel part of packet size adjustment, \fBoverhead\fR will be added
to original size, and then slot will be calculated. If the size would cause
overflow, more than 1 slot will be used to get the final size. This of course
will affect accuracy, but it's only a guard against unusual situations.

Currently there are two methods of creating values stored in the size table \-
ethernet and atm (adsl):

.IP ethernet 4
.br
This is basically 1\-1 mapping, so following our example from above
(disregarding \fBmpu\fR for a moment) slot 0 would have 8, slot 1 would have 16
and so on, up to slot 127 with 2048. Note, that \fBmpu\fR\~>\~0 must be
specified, and slots that would get less than specified by \fBmpu\fR will get
\fBmpu\fR instead. If you don't specify \fBmpu\fR, the size table will not be
created at all (it wouldn't make any difference), although any \fBoverhead\fR
value will be respected during calculations.
.IP "atm, adsl"
.br
ATM linklayer consists of 53 byte cells, where each of them provides 48 bytes
for payload. Also all the cells must be fully utilized, thus the last one is
padded if/as necessary.

When the size table is calculated, adjusted size that fits properly into lowest
amount of cells is assigned to a slot. For example, a 100 byte long packet
requires three 48\-byte payloads, so the final size would require 3 ATM cells
\- 159 bytes.

For ATM size tables, 16\~bytes sized slots are perfectly enough. The default
values of \fBmtu\fR and \fBtsize\fR create 4\~bytes sized slots.
.PP
.
.SH "TYPICAL OVERHEADS"
The following values are typical for different adsl scenarios (based on
\fB[1]\fR and \fB[2]\fR):

.nf
LLC based:
.RS 4
PPPoA \- 14 (PPP \- 2, ATM \- 12)
PPPoE \- 40+ (PPPoE \- 8, ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
Bridged \- 32 (ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
IPoA \- 16 (ATM \- 16)
.RE

VC Mux based:
.RS 4
PPPoA \- 10 (PPP \- 2, ATM \- 8)
PPPoE \- 32+ (PPPoE \- 8, ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
Bridged \- 24+ (ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
IPoA \- 8 (ATM \- 8)
.RE
.fi
There are a few important things regarding the above overheads:
.
.IP \(bu 4
IPoA in LLC case requires SNAP, instead of LLC\-NLPID (see rfc2684) \- this is
the reason why it actually takes more space than PPPoA.
.IP \(bu
In rare cases, FCS might be preserved on protocols that include Ethernet frames
(Bridged and PPPoE). In such situation, any Ethernet specific padding
guaranteeing 64 bytes long frame size has to be included as well (see RFC2684).
In the other words, it also guarantees that any packet you send will take
minimum 2 atm cells. You should set \fBmpu\fR accordingly for that.
.IP \(bu
When the size table is consulted, and you're shaping traffic for the sake of
another modem/router, an Ethernet header (without padding) will already be added
to initial packet's length. You should compensate for that by subtracting 14
from the above overheads in this case. If you're shaping directly on the router
(for example, with speedtouch usb modem) using ppp daemon, you're using raw ip
interface without underlying layer2, so nothing will be added.

For more thorough explanations, please see \fB[1]\fR and \fB[2]\fR.
.
.SH "ETHERNET CARDS CONSIDERATIONS"
.
It's often forgotten that modern network cards (even cheap ones on desktop
motherboards) and/or their drivers often support different offloading
mechanisms. In the context of traffic shaping, 'tso' and 'gso' might cause
undesirable effects, due to massive TCP segments being considered during
traffic shaping (including stab calculations). For slow uplink interfaces,
it's good to use \fBethtool\fR to turn off offloading features.
.
.SH "SEE ALSO"
.
\fBtc\fR(8), \fBtc\-hfsc\fR(7), \fBtc\-hfsc\fR(8),
.br
\fB[1]\fR http://ace\-host.stuart.id.au/russell/files/tc/tc\-atm/
.br
\fB[2]\fR http://www.faqs.org/rfcs/rfc2684.html

Please direct bugreports and patches to: <netdev@vger.kernel.org>
.
.SH "AUTHOR"
.
Manpage created by Michal Soltys (soltys@ziu.info)
